Automated vehicles and guidance systems associated therewith have been commercially developed whereby the vehicles effectively "track" a predetermined guidepath comprising either guide lines on a floor surface or guide wires buried under a floor surface. Exemplary vehicles and guidance systems are disclosed in commonly assigned U.S. Pat. Nos. 3,935,922; 3,970,840; and 4,003,445. The vehicles disclosed in these patents can be adapted for specific uses such as floor maintenance, mail delivery, and similar functions routinely performed within industrial and commercial organizations.
In the commonly-assigned Hainsworth, et al, U.S. Pat. No. 4,379,497 issued Apr. 12, 1983, a vehicle collision avoidance system comprises unmanned vehicles which are made to follow a predetermined guidepath, wherein the guidepath comprises fluorescent markings on the path surface. The vehicles employ light detection systems to maintain alignment along the path. Automatically guided vehicles which can be programmed to follow a predetermined course responsive to coded instructions on the floor surface are also well known. For example, DeLiban, U.S. Pat. No. 3,147,817 issued Sept. 8, 1964; Paulus et al, U.S. Pat. No. 2,317,400 issued Apr. 17, 1943; and Kohls, U.S. Pat. No. 3,411,603 issued Nov. 19, 1968. Each of these systems employs a buried wire type guide path. Another type of system for providing automatically guided vehicles utilizing buried wires is shown in the DeLiban et al, U.S. Pat. No. 4,284,160.
Automatically guided vehicles using reflected light and photo cells to detect the position of a reflected light guideline are disclosed in the Vischulius, U.S. Pat. No. 3,718,821 issued Feb. 27, 1973; Kudo, U.S. Pat. No. 3,738,433 issued June 12, 1973; Ando, et al, U.S. Pat. No. 3,881,561 issued May 6, 1975; and Wesener, U.S. Pat. No. 3,628,624 issued Dec. 21, 1971.
The general art of providing automatically guided vehicles following predetermined guidepaths employing buried wire systems or similar types of guidepaths has been relatively established for nearly two decades. In their simplest form, buried wire guidance systems pass electric currents through the guide wires which the vehicles are adapted to follow. The guided vehicles are made to follow these paths by incorporating various electrical components on the vehicles such as magnetic field sensing detectors. For example, the sensing detectors can include components having inductive elements in the form of an inverted "V" on the vehicle. The relative output of two inductive components comprising the legs of the inverted "V" can be used to provide a guidepath error signal to the vehicle's steering system.
Electric circuits on the vehicle can utilize the error signal to maintain the vehicle on the path. The concepts of automatically-guided vehicles having traffic paths defined by conductors energized so as to generate a substantial magnetic field, and also showing sensors on the vehicles to detect the magnetic fields and control steering are disclosed in U.S. Pat. Nos. 3,009,525 and 3,147,817. A system specifically showing a pair of sensing coils mounted on an automatically-guided vehicle and a system comprising an AC current-carrying conductor, wherein the magnetic field induces a voltage in the coils, is shown in the Ballantyne, U.S. Pat. No. 3,598,196.issued Aug. 10, 1971.
When the guidepath includes multiple paths, the vehicle can be made to differentiate between paths by having different frequencies of current generated through the various paths. Through conventional electrical circuitry on the vehicles, the vehicles can then be made to follow only currents of particular frequencies. Specifically, the means by which the vehicles can be maneuvered through the guidepath network can be either by switching a single frequency to which a vehicle responds onto one of several intersecting guide wires, or by otherwise exciting the several guide wires continuously and alerting the vehicle to change the frequency to which it is responsive. In the latter case, it is apparent that the vehicle will have substantially more sophisticated "intelligence" than in the former case, the intelligence being in the form of programming control or the like. Systems disclosing the concept of an automatically guided vehicle being responsive to a particular frequency and systems employing a guide path wherein each of the guide wires is energized at a different frequency are shown in the Hosking, et al, U.S. Pat. No. 3,039,554, and the Waites, U.S. Pat. No. 4,010,409 issued March 1, 1977. The Waites patent describes a frequency selective vehicle guidance system wherein the vehicle frequency discrimination circuitry uses phase-locked loop and other conventional circuits to provide signals representative of deviation from a guidance cable carrying a particular frequency.
Systems as described above can be characterized as "buried magnetic-field" systems. These buried magnetic-field systems have many advantages, including relative simplicity and relative "ruggedness" in various types of industrial environments. However, several problems also exist with respect to such systems. For example, it is possible that wire breakage can occur at flexing floor sections, such as expansion joints and floor cracks. Because of the necessity for a low ohmic resistance in the wire, such wires must typically be of a solid or stranded metal construction. Solid or stranded metal wires are relatively unbendable and not readily stretched so as to accommodate flexion.
In addition, such wires are often subject to corrosive chemicals, especially when such vehicle guidance systems are employed in industrial environments. In fact, some of these corrosive chemicals can actually be within the floor material itself. Still further, in vehicle guidance systems wherein the magnetic field generated by a current flowing through the wires is detected by the vehicles, each guide path wire segment must be a closed current loop, with the current adjusted so as to be between certain predetermined values. In multiple path systems, the requisite wire layout can be extremely complex, and can require a substantial amount of additional wire length which is not actually used for guidance.
Also, as shown by the prior art, known guidance systems having multiple guidepaths typically require current generation at several different frequencies. In a relatively complicated guidepath network, the requirement of multiple frequency generation can result in substantial difficulty in arranging a wire network in which the individual wire guidepaths can be distinguished from each other.